I just went through the same ordeal. CLP, Bore-tech Bore Cleaner, Kroil, hydrogen peroxide/vinegar. Tried soaking overnight with the first three. Tried all four individually in a ultrasonic tank. No magical cure found.

There are a few issues with the Endomags.
The ejector is too soft. Occasionally it would move in front of the bolt and jam it up. I'm thinking of either applying some RTV or epoxy to stiffen it up, making it almost permanent installation.
2nd issue is even downloaded to 28 rounds, it is super hard to reload with the bolt forward. If the ejector moves on you in the mag pouch, you will never get it to lock.
Since the ejector is on the mag, ULSC is a different process. You need to leave the mag in place, lock the bolt back, then remove the mag. If you don't do that, you have to manually remove the round from the bolt.

There are sites you can download the WRC. PM me if you want them. Don't want too many people with knowledge of the site because FIA might get a hold of it and shut them down or they run out of bandwidth.
WRC will effectively be dead at the end of this year. It's down to Citroen and Ford. Suzuki and Subaru both pulled out at the end of last year. Next year, they are going to use the Super 2000 rules which effectively ends the WRC cars.